The construction of mid- to high-rise building structures has progressed dramatically over the past decades, with advances in materials, techniques, and analytical methods that enable the economical construction of structurally sound multistory buildings. High-rise structures have many advantages, particularly in an urban setting, including, for example, more productive use of limited land space, economies of scale for building owners and managers, relatively low construction costs per square foot of usable space, attractive and desirable living and/or working space for users, and reduction in urban sprawl for municipalities.
A construction method of choice, particularly in structures more than a few stories tall, utilizes a concrete shear core that functions as a primary structural element for the building. The concrete shear core is essentially a large, hollow, vertical column of reinforced concrete, located generally at an interior location in the building. The concrete shear core typically is a hollow rectangular column that extends along the entire height of the building. The concrete shear core provides a sturdy central structural member that, cooperatively with peripheral columns and transverse beams, reacts to the static and dynamic loads imposed by and on the building. The concrete shear core often houses many of the building services, such as the elevators, utilities, and the like.
Advantages of the concrete shear core construction are well known. For example, in a concrete shear core building, no structural steel bracing or moment connections are required, which are expensive and may interfere with functional and aesthetic aspects of the building. Also, the concrete shear core construction significantly reduces the need for structural steel per square foot of built space, while providing relatively large, column-free tenant space. The concrete shear core provides economic advantages in part because in-place rebar and concrete are cheaper than structural steel. The concrete shear core allows the perimeter structural steel columns to be relatively light, gravity-loaded columns only. The horizontal forces, e.g., forces generated from wind or earthquakes, are resisted substantially by the rigid concrete shear core. A well-designed concrete shear core construction provides a high performance, stiff building that is able to withstand dynamic loads, such as winds and the like, without producing undesirable motion that can be disconcerting to occupants of the building.
There are disadvantages to conventional concrete shear core construction, however. In particular, the construction of a conventional concrete core is relatively time-consuming and has a significant impact on the total time required to complete a building. The time required to complete the construction of a building is extremely important to the overall cost of the building and, therefore, reducing the total construction time is an important goal for controlling the overall cost of construction. The concrete shear core, constructed using conventional methods, typically requires approximately one week per floor to erect. Time-consuming steps required for the concrete shear core construction include moving the forms from floor to floor, setting the reinforcing bars in place (with appropriate overlap at each floor), pouring the concrete, and allowing the concrete to set. The corresponding structural steel and floor construction, in contrast, requires only about two to three days per tier (two floors) to erect. The peripheral structural steel relies on the concrete shear core for support and, therefore, in prior art construction methods, the peripheral steel structure must be erected only after the corresponding portion of the concrete shear core has been completed.
Because the per-floor time required to construct the concrete core is significantly greater than the per-floor time for the steel and floor assembly, often the steel erection work will be performed in stages. For example, after the concrete shear core reaches a desired height, the structural steel and floor work may begin and proceed simultaneously with the concrete shear core construction. When the structural steel construction catches up to the current progress on the shear core, the steelwork must be temporarily halted while the shear core is extended further. When the concrete shear core is completed to a second desired height, a second stage of steel construction may be started. Staging the steel erection work requires mobilization and demobilization of the steel-working equipment and work force, thereby further increasing the cost of construction.
Other disadvantages to conventional concrete shear core construction are that it typically requires rebuilding the core forms at story height transitions or using expensive self-climbing forms, and requires redundant construction equipment, such as temporary stair towers and hoists.
Referring now to FIG. 1, there is depicted a concrete shear core building at an intermediate phase of construction, using prior art construction techniques. In a conventional concrete shear core construction, after the building site is prepared, construction of the concrete shear core 50 begins. As the concrete shear core 50 is extended, forms—for example, plywood forms or so-called “climbing forms” 52—are positioned to define the concrete shear core walls, that is, the volume to be filled with reinforced concrete, typically an annular square cylindrical volume. Steel reinforcing members or rebar (not shown) are then placed vertically in the defined volume. Horizontal steel beams or attaching members 51 are also positioned to be partially embedded in the concrete. Typically, one floor (approximately 10-16 feet in height) is poured at a time. In order to achieve adequate tensile load transfer between vertical sections, the rebar extends above the level that the concrete is to be poured, permitting adjacent floors to have overlapping sections of rebar. An overlapping length of about six feet is typical.
The concrete is then poured to the desired height within the volume defined by the forms 52 and permitted to set. The forms 52 are then moved up to the next floor. This may require the construction of supports for the forms and/or modification of the forms to define the next volume to be filled with concrete. Additional rebar and horizontal support members are then installed, and additional concrete is poured to the next desired height. This process is repeated to complete the concrete shear core. Using conventional methods, it takes about one week to construct one floor of the concrete shear core, depending on the configuration.
Once the concrete shear core is completed to a predetermined height, steel columns 54 are erected generally about the design perimeter of the building and at intermediate locations as required. Horizontal beams (not visible in FIG. 1) are installed between the columns 54 and/or the concrete shear core 50. Corrugated horizontal steel panels are then installed, supported on the beams, and concrete is poured onto the steel panels to define each floor 56. It typically takes approximately two to three days to complete one tier of steel and floor construction. The concrete shear core construction, therefore, is typically a pacing task, directly impacting the time required for constructing taller buildings. If the concrete shear core construction can be accomplished more quickly—for example, at a pace similar to the construction of the steel structure—then the total time required to complete the building may be substantially reduced.
There remains a need, therefore, for a construction method for mid- and high-rise buildings that retains the advantages of the concrete shear core design while improving the speed and efficiency of constructing the concrete core.